1. Field of the Invention
The present invention relates to an inverter outputting an alternating voltage, a dielectric-barrier discharge generation device using the inverter as power source, and a sheet material modification apparatus using the dielectric-barrier discharge generation device.
2. Description of the Related Art
High-voltage inverters outputting an alternating high voltage are widely used as discharge power source devices or the like for plasma generation devices, ozone generation devices or the like.
For example, atmospheric plasma is used as a means of surface treatment in various industrial products to modify the surface material or eliminate contaminants from the surface or the like. When resin or the like is to undergo a bonding, printing, coating process or the like, pretreating the resin by atmospheric plasma makes it possible to improve the surface in wettability and easily achieve a favorable finish by the bonding, printing, coating or the like.
For example, Japanese Laid-open Patent Publication No. 2011-57442 suggests such a modification apparatus. At the modification apparatus, a discharge electrode roller and counter electrodes are opposed to each other across a carrying belt, a pulsed high voltage is applied to between the roller and the electrodes to generate dielectric-barrier discharge due to creeping discharge. Accordingly, atmospheric plasma is generated and brought into even contact with a surface of the sheet material to be processed.
In such a process of modifying the surface of the sheet material, to produce stable dielectric-barrier discharge for generating atmospheric plasma, it is necessary to stably supply high power resulting from an alternating high voltage of several KV to several tens of KV to between the discharge electrodes and the counter electrodes opposed to each other across a dielectric as a load.
Japanese Laid-open Patent Publication No. 2012-191828 discloses a high-voltage inverter (device) suitable as such a power source, for example.
The high-voltage inverter switches an input voltage in which a pulse flow is superimposed on a direct current or a direct current component to flow an excitation current to the primary excitation winding of a transformer and output an alternating high voltage from a secondary output winding.
The “alternating high voltage” here refers to not a sinusoidal alternating high voltage uniform in positive and negative poles but a high voltage with a pulsed or pulse-flow alternating waveform that results from a flyback pulse generated on the output winding due to intermittence of an excitation current from the transformer.
Such a conventional high-voltage inverter is basically configured as illustrated in FIG. 12. The high-voltage inverter 100 includes a transformer 13 that has as input voltage Vin a direct-current voltage (possibly including a pulse-flow component) obtained by rectifying and smoothing an alternating voltage from a commercial power source 11 by a rectifying/smoothing circuit 12; a switching element 14 formed by FET or the like; and a control circuit 15.
The transformer 13 has excitation winding Np and output winding Ns. The excitation winding Np is connected to a feed circuit from the rectifying/smoothing circuit 12 in series with the switching element 14. The switching element 14 is on-off controlled by a switching signal Sp output from the control circuit 15 to a gate terminal thereof.
The switching signal Sp applied from the control circuit 15 to the gate of the switching element 14 has a rectangular wave as illustrated in FIG. 13(a). The low-level period of the switching signal Sp is the OFF period, and the high-level period of the same is the ON period. Reference sign T denotes one period.
The high-voltage inverter 100 illustrated in FIG. 12 is a flyback voltage resonant inverter. Therefore, input voltage Vin is switched by the switching element 14 to turn on or off an excitation current to be flown into the excitation winding Np of the transformer 13. In the ON period, excitation energy is stored in the excitation winding Np, and in the OFF period, the energy is discharged from the output winding Ns of the transformer 13 to output to a load 2 output voltage Vout of the waveform as illustrated in FIG. 13(b).
Therefore, the output voltage Vout is a high voltage according to the turn ratio of the excitation winding Np to the output winding Ns, and has a half-wave type of an approximately sinusoidal waveform as illustrated in FIG. 13(b). In this example, the output voltage Vout is a positive (+) voltage equivalent to a positive half wave, but may be a negative (−) voltage equivalent to a negative half wave.
A voltage of such a waveform or a waveform close to the same will be referred herein to as alternating high voltage.
However, the output voltage Vout of the high-voltage inverter 100 does not actually have a proper half-wave sinusoidal waveform as illustrated in FIG. 13(b).
The output voltage Vout is generated by a parallel resonant circuit including inductance Ls of the output winding Ns, and composite capacity C of distributed capacity Cs of the output winding Ns and equivalent capacity (load capacity) Co in the load 2.
Then, the output voltage Vout alternating at a high voltage generates a strong magnetic field in its output path. As a result, as illustrated in FIG. 14, a steep transient current flows as output current Iout at ON/OFF switching of the switching element 14, and immediately after that, an oscillating transient voltage (ringing) is generated.
FIG. 15 is an enlarged view of the waveform of the output voltage Vout enclosed in dotted circle A in FIG. 14 immediately after the switching element 14 turns off.
Such a transient voltage results from a surge voltage generated at ON/OFF switching of an excitation current and an attenuated oscillating voltage, and may reach 10 to 20% of crest value of the output voltage Vout even at stable operation time.
The transient voltage leads to a decrease in fundamental high value as an inverter output generated at the output winding in the OFF period of the excitation current, and dispersion of output energy, which may be excessive output not used in a load.
For example, when the load 2 illustrated in FIG. 12 is a dielectric-barrier discharger and the dielectric-barrier discharge start voltage is 6 KV, portions of the output voltage Vout falling below 6 KV do not contribute to dielectric-barrier discharge.
Accordingly, the generation of oscillating transient voltages as described above lead to reductions in efficiency and reliability of the inverter, heat generation, and the like. Further, there is a possibility that electromagnetic noise is dispersed to cause radio disturbances at peripheral devices.
The foregoing problems become more pronounced with a higher output voltage of the inverter. However, no effective measures have been taken on high-voltage inverters in which the output voltage is an alternating high voltage of several KV to several tens of KV.
To absorb surge currents and transient voltages generated at the primary side of a transformer, it is known to provide a snubber circuit composed of a resistor or an inductor, a capacitor, a diode, and the like.
However, when a snubber circuit is provided at the secondary side of a transformer in a high-voltage inverter as described above, a desired output voltage cannot be obtained due to a resonance shift. The components of the snubber circuit need to have pressure resistance about three times that to the output voltage. However, in the case where the output voltage is several tens of KV, it is difficult to obtain existing components with such pressure resistance. Thus, no snubber circuit can be virtually provided at the secondary side of a transformer.